1. Field of the Invention
The present invention relates to a device for extraction of a partially defined sample volume from a larger liquid sample volume, to methods for determining the volume of at least one cavity of a device, to a set of at least two devices for dilution of liquid samples to the same or different defined degrees, to an analytic instrument for operative connection to a sample measuring and diluting device having at least one measuring cavity, and to a system having a sample measuring and diluting device and an analytical instrument.
2. Description of Related Art
In one type of blood testing, a system comprising a blood cell counting apparatus/analytical instrument provided with a blood sample volume defining device, preferably in the form of a disposable cassette, is used. To be able to count the blood cells in an accurate and repeatable way it is of crucial importance to define an accurate volume of a blood sample. The accurately defined volume of blood sample is normally diluted by an accurately defined volume of a diluent or a lysing agent, in order to obtain a dilution ratio of typically 1:100 to 1:80000. When counting white blood cells is concerned the dilution ratio is typically 1:400 and when counting red blood cells is concerned the dilution ratio is typically 1:40000, in the latter case the dilution often taking place in two steps. Thus, it is obvious that variations in sample volumes and dilution liquid volumes must be minimized such that a correct degree of dilution and counting of blood cells always can be guaranteed.
Apparently, obtaining the same sample volumes are a critical step in the dilution procedure, since the volumes concerned are extremely small compared to the corresponding diluent volumes. To be able to obtain an accurate dilution ratio of a blood sample it is of crucial importance that the cavity in which the blood sample is contained is filled in a very precise and accurate and repeatable way, and that the variations regarding the volume between different cavities having the same nominal volume are made as little as possible.
The Swedish patent application No. 0303157-2 describes a method and a device for defining a small volume of a larger liquid sample. The device includes a first body and a second body. In a surface thereof, the first body has at least one cavity having said defined volume, and the second body includes an edge relatively slidable along said surface and over the cavity thereby separating an excessive volume of sample from said surface, leaving a defined volume within the cavity.
By the non-published Swedish patent application No. 0500784-4 a sample volume defining device is known which comprises a sledge and a framework, movable relative to each other. The device forms a part of a disposable support, preferably in the form of a cassette, and the framework is formed integrally with said support. The support comprises at least two chambers, one of which is filled with an accurately defined volume of diluent or lysing agent for dilution of a liquid sample, preferably a blood sample, and the other is empty and thus used for achieving the dilution and mixing of the liquid sample. At least one accurately defined cavity is made in the sledge. The components of the device are preferably made by injection moulding.
Although all due care is take when said at least one cavity is made in the first body/sledge, which in the description below is called a sampling body/body, its volume will on one hand differ from sampling body to sampling body made by the same moulding tools as the tools wear, and so forth, and on the other hand from moulding tools to moulding tools. By knowing this volume variation between different cavities made in different sampling bodies, it is thus possible to even further improve the accuracy of the blood analyses when performing blood cell counting.